Optical systems characterized by a relatively narrow field of view, such as refracting telescopes and various long-range viewing apparatus of similar design, have a number of characteristic limitations affecting optical performance. Among these is chromatic aberration, largely due to the dispersive properties of glass. Dispersion of an optical substrate causes the refraction of a lens to vary with wavelength, so that, for conventional optical glass, the index of refraction is higher for short wavelengths and decreases as wavelength increases. As a result of chromatic aberration, blue light rays come to focus closer to the lens than do red light rays.
The problem of chromatic aberration in telescopes has been addressed in a number of ways. Use of a very slow focal ratio (high ratio of focal length to lens diameter or f-number), on the order of f/150 reduces the severity of the aberration but results in devices that are extremely long and, at best, mechanically cumbersome. In the Newtonian reflecting telescope, mirrors are used in place of lens elements in order to reduce chromatic aberration; however, this approach tends to increase a number of monochromatic aberrations.
Different types of optical glass exhibit different dispersive properties, conventionally specified in terms of Abbe number or in terms of partial dispersion. For each type of glass, optical glass manufacturers provide Abbe number and partial dispersion information, along with refractive index values at one or more standard wavelengths. Crown glass has relatively low dispersion and high Abbe number. Flint glass has relatively high dispersion and low Abbe number. Taking advantage of these differences, telescope objective designers developed the achromatic doublet that combines low and high dispersion glass. This solution provides a measure of correction for chromatic aberration, allowing optical systems with lower f-number, on the order of f/15, and correspondingly reducing the required length of the telescope device. With a well-designed achromatic doublet, the focal points for red and blue light become substantially coincident, but still differ from the focal point for green light. This leaves some amount of chromatic aberration, termed secondary color.
Further development and understanding of glass types with anomalous dispersion properties has helped to improve imaging performance by reducing chromatic aberrations. Apochromatic doublets using anomalous dispersion glass types have allowed focal ratios to shrink further, to about f/10. With the addition of a third element, apochromatic triplets extended the focal ratio range to about f/8 and, in addition, help to reduce monochromatic aberration. Addition of more elements and the development of superachromats have provided correction that reduces aberration to very low levels and improved image quality accordingly, at higher cost and complexity.
Significantly, more highly anomalous glass types can be particularly effective for use in color correction. At the same time, however, these highly anomalous glass types are increasingly expensive and can be difficult to fabricate.
Solutions for telescopes providing near-apochromatic performance using common optical glass have been proposed, such as those described in International Publication No. WO 2006/091181 by Duplov. Duplov presents a 5- to 7-element design that provides some level of apochromatic performance using only normal glasses. This design, however, does not provide well-behaved control of color. Significantly, the Duplov arrangement requires that the track length of a telescope that is designed using these optics must be from about 1.75 to 2.5 times the focal length, which is disadvantageous.
Thus, it is seen that there is a need for optical designs for telescopes and other afocal optical apparatus that provides a high level of color correction without the cost and dimensional requirements of earlier solutions.